1. Field of the Invention
The invention relates to a control method for a solvent refining process. More particularly, the invention relates to solvent refining petroleum derived lubricating oil stocks to yield aromatics-lean raffinates and aromatics-rich extracts. Most particularly the invention relates to simultaneous control of both raffinate and extract quality.
2. Description of the Related Arts
It is well-known in the art to upgrade lubricating oil stocks. Upgrading typically involves treating these stocks with selective solvents to separate a relatively more aromatic fraction from a relatively more paraffinic fraction. In such a treatment, the preferred configuration comprises a countercurrent extraction process in which the lighter lubricating oil phase is introduced into the center or bottom section of the countercurrent extraction tower. The oil phase flows upwardly through the extraction tower and contacts downwardly flowing solvent which is introduced into the upper section of the extraction tower. A relatively paraffinic fraction, termed raffinate, is recovered from the top section of the extraction tower while solvent and relatively aromatic fraction, termed extract, is recovered from the bottom section of the tower.
Extract is used commercially as a rubber extender and processing oil. Nonaromatic content is the primary measurement of quality.
Multistage solvent extraction processes are also known wherein either the raffinate phase, the extract phase or both are subjected to repeated extraction to enhance a desired property.
U.S. Pat. No. 4,866,632 to T. C. Mead et al. teaches a control means and method for a solvent refining processing unit. An algorithm and control system are provided for optimizing the flow of charge oil to provide the maximum yield of extracted oil of a specified quality, measured by refractive index. The invention is based on the discovery that when a charge oil is refined to yield a raffinate of given refractive index, the raffinate viscosity will be the same regardless of the refining temperature and solvent dosage.
U.S. Pat. No. 4,053,744 to R. A. Woodle teaches a control means for a solvent refining unit. The temperature of the extract mix in the solvent refining tower, the flow rate of the charge oil, the flow rate of the solvent and the flow rate of the extract oil are sensed and corresponding signals provided. The control means is operated in accordance with the signals to achieve either a maximum allowable flow rate for the solvent; a maximum allowable flow rate for the extract oil; a maximum allowable flow rate for the refined oil or a reduced charge oil flow rate for a fixed refined oil flow rate.
U.S. Pat. No. 4,328,092 to A. Sequeira, Jr. teaches a process for the solvent extraction of hydrocarbon oils. In the process N-methyl-2-pyrrolidone is the extraction solvent. The hydrocarbon oil is solvent extracted to form two phases, a secondary extract phase and a secondary raffinate phase. The secondary raffinate phase is returned to the extraction zone. As a result, an increased yield of refined oil product and a savings in energy is achieved.
U.S. Pat. No. 4,304,660 to A. Sequeira, Jr. discloses lubricating oils suitable for use as refrigeration oils. Those lubricating oils are produced by solvent extraction of naphthenic lubricating oil base stocks to yield an extract which is mixed with a solvent modifier and cooled to form a secondary raffinate and secondary extract. The secondary raffinate is treated with concentrated sulfuric acid and caustic neutralized to produce the refrigeration oil.